(1) Field of the Invention
This invention relates generally to electronic power supply and is specifically applied in any dual supply rail systems, which require a smooth and uninterrupted output supply as e.g. in dual input supply generation of electronic real time clocks (RTC).
(2) Description of the Prior Art
The real time clock (RTC) supply domain is an essential feature on most of the Power Management (PM) ICs and typically incorporates the following blocks and functionality—crystal (XTAL) oscillator, digital block with control, timer and alarm functions, power-on-reset (POR), and input/supply detection circuits. There are two important requirements for the power supply generation and the power consumption of the RTC domain:                ultra-low (less than 5 uW) power consumption from the back-up power source (coin cell or super capacitor)        zero current from the back-up source in the presence of the main system supply (external charger or main battery)        
One popular approach is to power the RTC domain directly from the back-up source. This simple implementation is very attractive but unfortunately does not satisfy the second requirement because of the constant (even when system supply is present) discharge of the back-up power source. To overcome this problem, designs based on this concept have to keep the back-up battery charger permanently on to top up the back-up energy source, which in its turn increases the overall IC power consumption.
Another existing practice is the integration of a supply comparator that monitors the levels of the system rail and the back-up source and connects the RTC power supply to the higher of the two rails. This more power efficient solution though, creates a major design problem. The main principle of operation (connection to the highest supply rail) and the large variation in the voltage levels of the different power sources (4-5.5V external charger, 2-4.5V main battery and 1.5-3.3V back-up battery) implies that the resultant RTC supply voltage level in the extreme case might vary between 1.5V and 5.5V. The implementation of a XTAL oscillator or digital control logic able to operate in such a wide supply range and to sustain the abrupt changes from min to max levels is not a simple design task. Such designs require complex and not necessarily power efficient circuit implementations that are costly in terms of design time and silicon area.
It is a challenge for engineers to provide an alternative method and circuit for power efficient generation of the RTC supply rail that does not have the disadvantages of the existing solutions.
There are known patents or patent publications dealing with RTC power supplies:
U.S. Patent (U.S. Pat. No. 6,016,045 to Thomas et al.) discloses a back-up battery system for use with a real-time clock of an on-board device controller. In its normal operation, the real-time clock (RTC) is powered by a primary power source, such as a vehicle battery. When the primary source is interrupted, the back-up battery system is invoked to provide back-up power to the RTC. The system includes a low voltage dry cell battery mounted within a fuse component configured for electrical engagement within the vehicle fuse block. In one embodiment, a relay is interposed between the fuse component and the RTC to make and break the back-up power circuit in response to the state of the primary power source. In another embodiment, the battery within the fuse component is replaceable, with the fuse component including a base and a removable cap. In still another embodiment, the battery within the fuse component is a rechargeable battery, and the back-up power system includes a recharging circuit operable with the primary power source.
U.S. Patent (U.S. Pat. No. 7,550,954 to De Nisi et al.) discloses a versatile voltage regulator accommodating either an Alkaline or Lithium-Ion battery main battery and providing low-current power for a real time clock module and for charging a backup battery. Depending upon the battery power source that is used, the present invention provides a best circuit configuration for efficient power conversion. If the power converter according to the present invention provides a regulated output voltage that is greater than the main battery voltage of an alkaline battery, a low drop-out-voltage (LDO) voltage regulator is used in feedback loop with a charge pump. Otherwise, for a Lithium-Ion battery, only a LDO voltage regulator is used. The voltage regulator includes a series low drop-out-voltage (LDO) voltage regulator that is coupled between the main external battery and the Vout load terminal, when the voltage at the Vout load terminal is less than the voltage of the main external battery. The voltage regulator also includes a charge pump circuit adapted to be connected in feedback loop with the LDO regulator and the Vout load terminal, when the voltage at the Vout load terminal exceeds the voltage of the main external battery.
U.S. Patent (U.S. Pat. No. 4,698,530 to Thomson) proposes a power switching circuit for automatically switching between line-driven and battery power supplies. The power switching circuit selectively connects first and second input voltage terminals Vdd and Vbb to an output voltage terminal. When the line-driven power supply is on, a first transistor switches on to connect the first input voltage terminal to the output voltage terminal, and a second transistor switches off to isolate the battery. When the line-driven power supply is off, the first transistor switches off, and the second switches on to connect the battery powered second input voltage terminal to the output voltage terminal.